1. Field of the Invention
The present invention relates to a system for transmitting digital signals including data blocks, each of the blocks being delimited by one of many possible words (DDi) that are introduced in the transmission side and are reconstructed in reception, the blocks being located in transmission according to an enumeration that must be reconstructible in reception according to the received composite signal, even in the case in which transmission line errors can cause the data block delimiting words to switch with one another or destroy some of these words and/or introduce untransmitted delimiting words.
2. Description of the Related Art
Many applications of numerical transmission require the transmission of data blocks, these blocks being disposed according to an ordered succession (e.g. according to the integer ciphers 0, 1, 2, 3 . . . ) and this ordering must be stored and recognized in the reception side, even in presence of errors in the data transmission.
Many applications require data blocks that have a randomly variable dimension (bit number) and the single data of each block are coded with a variable length e.g., by Huffman's codes; the delimiting of data blocks uses some well-known technique, such as the use of synchronization words or the use of data block delimiting words (in the following, these delimiting words are indicated as DDi, with i being the integer cipher 0, 1, 2, . . . i . . . n-1, n).
The synchronization words have the drawback of having a high bit number to avoid being interpreted as words or word sequences of data blocks in presence of transmission errors. Therefore, a synchronization word is used for a certain number of data blocks and further, if necessary, the length information (expressed in bit number) of the blocks that follow the synchronization word is sent into the transmission line.
When DDi words are used, these words are located at the end of each data block, and when more than one DDi word is used, various DDi words are used so that it is possible to recognize, in addition to the limit of data blocks, the grouping limits of data blocks. To better explain the problem, reference is made to video signal coding by the use of discrete cosine transforms (DCT).
Considering the sole luminance signal and considering the video signal in digital form, the points in each part of a video frame are grouped in square blocks of 8.times.8 points; the DCT transform is applied to each square block of points and the resulting values are coded in a variable length after a precision reduction. To separate the data related to each initial block of 8.times.8 image points, DD words can be used in addition to some information that identifies the first 8.times.8 block in each part of the frame.
On the reception side, the 8.times.8 blocks of reconstructed image data must be placed in their proper position in the video signal frame. Errors in the transmission of the video signal (or residual uncorrected errors, when the transmitted data are transmitted with error correction codes) can have the effect of destroying some DD words or transforming some data related to the points of 8.times.8 block into DD words, thus making it impossible to reconstruct the data blocks near the lost or erroneously created DD words, and impossible to properly position the image of 8.times.8 blocks following the erroneously separated blocks.
The above mentioned drawback can be overcome by transmitting synchronization words at the start of the television frame and using more than one DD word, so that a DDO word is used at the end of each data block, while a DDl word replaces the DDO word every four data blocks, and so on. Greater details on the use of DD words can be found, e.g. in the article of S. Cucchi and F. Molo entitled "DCT Based Video Codes for DS3 Digital Transmission" presented at the SMPTE Technical Conference, Oct. 14-19, 1988.
This system is advantageous in many but can be further improved. The object of the present invention is to obtain a more accurate reconstruction of the correct ordering of data blocks, when its separation requires the DD words, this reconstruction being made in presence of transmission errors.